This invention relates to an on-board sensor and method for using the same to measure the volatility of a sample of gasoline by measuring the change in capacitance of a sensing element as a function of time and temperature and using those measurements to estimate the driveability index of the sample.
It is known in the art relating to automotive engines, that the key gasoline characteristic for good driveability is volatility. Volatility is especially important at the time an engine is started because liquid gasoline must evaporate and mix with air to form a combustible mixture. If too little gasoline is added, the engine will not start; if gasoline beyond that needed to initiate combustion is added, then extra hydrocarbons from an unburned portion of gasoline are found in the exhaust. Moreover, because gasoline sold in the United States varies in volatility, there is a tradeoff in engine design between low hydrocarbon emissions and good driveability with low volatility fuel.
To describe the effect of gasoline volatility on the cold start and warmup driveability of a vehicle, a driveability index (DI) has been developed. For gasoline that does not contain oxygenates such as ethanol or methyl tertiary-butyl ether (MTBE), the definition of DI is based on a laboratory test (ASTM D 86) in which a sample of gasoline is distilled as its temperature is raised. The fraction distilled is measured as a function of temperature and
xe2x80x83DI=1.5T10+3T50+T90
where Tx is the temperature in degrees Fahrenheit at which x% of the gasoline sample has been distilled.
Experiments have shown that even if DI is held constant, the presence of oxygenates in a fuel changes the cold start and warm-Lip driveability of a vehicle. With oxygenated gasoline, an expression that provides better correlation to engine performance is the New Driveability Index (NDI):
NDI=DI+43.2xcex4MTBE+86.2 xcex4EtOH
where the variables xcex4MTBE and xcex4EtOH are 1 if about 15% methyl tertiary-butyl ether or 10% ethanol, respectively, is present, and zero otherwise.
Although both DI and NDI are defined in terms of a laboratory procedure, they can also be estimated. One known way to estimate DI is by measuring the fuel""s infrared transmission spectrum. While this approach has proven useful in refineries where the feedstocks are known, it has not been accepted as an accurate way to characterize the DI of finished gasoline in the field.
It is particularly desirable to estimate DI/NDI on-board a vehicle. To provide customer satisfaction, engines are calibrated to reliably start with fuel of the lowest expected. This is done by increasing the amount of fuel in the air/fuel mixture. Consequently, for most starts, the engine""s air/fuel ratio is richer than optimum. Some of this extra gasoline passes unburned into the exhaust. This is particularly detrimental at the time of a cold start because the catalytic converter is too cold to be active. The added hydrocarbon concentration is typically emitted to the environment.
Estimating DI or NDI on-board would permit the air/fuel ratio to be more precisely controlled. The engine would be calibrated to reliably start while extra fuel would only be added when needed to compensate for fuel volatility. On the average, less fuel would be used for cold starts resulting in a decrease in fleet-average exhaust hydrocarbon emissions. This decrease in air pollution is an important environmental benefit.
The present invention provides an on-board sensor and method of using the same to determine or estimate DI (or NDI) by measuring changes in electrical capacitance of a fuel-filled sensing element as the sensing element is heated to evaporate the fuel within it.
While the engine is running, gasoline flows over a two-piece sensing element having a plurality of interdigitated plates that are arranged to retain a volume certain of gasoline between them after the engine is turned off Because the retained volume is controlled by the spacing between the interdigitated plates, the present invention eliminates the need to supply a precisely predetermined volume of sample to the sensing element for testing. In the preferred embodiment, a small amount of fuel (e.g. in the range of 0.04-0.1 ml) remains in the sensing element every time the engine is turned off.
The sensing element is then heated by means of a ceramic heater. The sensor""s change in capacitance and temperature over time is measured with circuitry operatively connected to the sensing element. Because the sensing element exhibits a relatively large change in capacitance (in the range of 4 pF) simple, relatively inexpensive circuitry may be implemented.
When the sensor reaches a predetermined level of capacitance, the heater is turned off and the measured data, which is representative of the volume and temperature of the sample, is sent to the microcontroller of the vehicle, which calculates DI, or NDI, as the case may be. The DI or NDI so calculated correlates well with laboratory calculated DI and NDI. Moreover, the measured change in capacitance over time between the full and empty states indicates whether or not the tested fuel contained ethanol. The calculated value of DI or NDI is then stored for the next cold start when it may be used for setting the desired air/fuel ratio at the time of starting.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.